Typically, in magnetic tape recording and reproducing apparatus, tape extends along a low inertia tape path from a supply reel or hub past recording/reproducing transducers and a capstan to a take-up reel or hub. The capstan engages the tape to provide bi-directional control of tape motion past the transducers. In magnetic tape recording, be it longitudinal or helical track recording, longitudinal tracks are utilized for providing some indication of timing as well as the location of the data recorded thereon. Traditionally, for playback, a clock is derived from the data coming from the tape with the use of a phase-locked loop circuit. The phase-locked loop attempts to track fluctuations in the data rate resulting from variations in tape speed. Ideally, a clock which is synchronous with the data is derived from the data itself. Phase detectors and phase-locked loops are required in this process.
Phase-locked loop circuits are non-linear feedback systems and require the use of loop filters. Phase-locked loop systems suffice in tape recording systems where clock phase is important and, more importantly, where the data rate, on both record and playback is nominally constant. However, where playback velocity in particular, varies over a very wide range, this makes it difficult to construct a phase-locked loop locking on to data being read from the tape over such a large range of data rates. Particularly at slow speeds, or where the tape is changing direction, control track information may not be reliably detected with an inductive head on a slow-moving tape since playback voltage is proportional to tape speed.
In present day recording systems, such as those which employ helical recording, for example, data is recorded on diagonal tracks or channels, physically positioned between two of three longitudinally recorded tracks or channels. By way of example, for digital video formats such as the D-2 format, and for digital data storage format known as DD-2, the data recorded in the longitudinal channels contains logical and physical information about the data recorded on the tape and needs to be recovered reliably at playback speeds ranging from 0.1 times normal to about 60 times normal. Fast search and retrieval operations are required, and the timing information relating thereto must be derived from the associated track, regardless of tape direction and/or speed. Phase-locked loop circuits operating over such a range, and deriving timing information from such tracks, are either unduly complicated, and hence expensive, or unreliable over the range.
For magnetic tape recorded on helical scan apparatus, the user data tracks (diagonal tracks) are generally recorded at high density, while the longitudinal tracks are generally recorded at low density. Longitudinal tracks are recorded with a fixed relationship to the user data tracks (as prescribed by the tape format). The information in the longitudinal tracks may contain either physical position information relating to the physical location at which user data is recorded on the tape, or logical position information relating to the physical location of logical structures in the user data on the tape. During search operations, user data can not be reliably decoded, so data from the longitudinal tracks is used to locate user data. The tape movement is thus typically controlled by relying on data from the longitudinal tracks.
Information on the longitudinal tracks is often encoded with self-clocking channel codes, which have at least one transition per bit cell. It is then possible to decode timing information derived from the data while detecting the data without a clock which is phase-locked to the data. One such method relies on measuring the time period between successive transitions in the data stream. Data is then detected from the sequence of measurements. For example, in Manchester codes, transitions in data bits are either one half bit cell or one bit cell apart. In the ANSI-standard sync marks used with these codes, two pairs of transitions are separated by three half bit cells. Thus, the ratio of longest time between transitions to shortest time between transitions is nominally restricted to a fixed ratio between transitions in the data stream. This constraint on allowable periods is used for synchronization and for establishing timing windows for the detection process. Based on measurements of bit cell widths in the past, thresholds are set up for time intervals. As new measurements are made, they are compared against these thresholds for the purpose of data detection. For example, based on the established timing windows, it is decided if the period between two transitions was one-half, one, or one and one-half bit cells long. As the tape slows down to come to a halt, bit cells are played back further and further apart in time, and the time intervals between bit cells may change significantly between transitions. If the demodulation clock used to measure the intervals does not change with tape speed, the measurement will result in erroneous detection.
In the prior art, there are instances of the use, in a video tape recorder, of a device coupled for rotation with, or in proportion to, the capstan to provide a signal related to movement of the capstan. One such apparatus is shown and described in U.S. Pat. No. 4,363,048, entitled "Time Counting Clock Generator", which issued to Tanaka et al. on Dec. 7, 1982. In the Tanaka et al. patent, there is disclosed a magnetic wheel coupled to the shaft of an idler roller or counter roller in physical contact with the magnetic tape. A pair of magnetic pick-up heads are provided in facing relation with the wheel, circumferentially displaced so that their output pulse signals are 90 degrees out of phase with respect to each other, that is, in quadrature. This patent describes a system to measure tape position, in which the measurement is used for position control of the tape and to compute the length of tape left on the reel. This quadrature position encoder is used to determine the direction of the tape travel and to provide a two-phase clock with a nominal frequency proportional to tape velocity. A circuit is used to synchronize the phase of the two-phase clock with pulses played back from a control track each time a pulse is encountered. The objective of the system is to measure tape length accurately, even when tape speed is slow or tape speed changes direction. It does, however, not generate a clock signal which can be used to detect data coming from the tape.
In accordance with one aspect of the present invention there is provided a timing recovery method and apparatus which derives and recovers the timing information without the use of phase-locked loops, or as a supplement to a phase-lock loop, and without relying on data rate information from the data itself.
In accordance with another aspect of the invention, timing is derived from tape velocity information available from the capstan servo, which tape velocity information is used to adjust the modulation clock frequency to aid in the detection of data.